Prior art workers have devoted much time and effort to the development of electrostatic dispensing nozzle assemblies to meet the demand of ever increasing applications in which a flowable material is to be uniformly applied over a predetermined area of a surface.
In general, an electrostatic dispensing nozzle assembly is provided with an elongated slot and an adjacent elongated dispensing edge. The fluid to be dispensed is distributed along the slot and provided in constant supply during the coating operation hydraulically (i.e. by mechanical means external of the nozzle such as a metering pump or the like). Means are provided within the nozzle to assure that the fluid is evenly and uniformly provided along the slot and the adjacent dispensing edge. Means are also provided within the nozzle (usually in the form of a metallic shim assembly) to apply an electrostatic charge on the fluid to be dispensed. Dropletization takes place after the fluid is introduced to the dispensing edge. The physical propulsion of the droplets to the target is the result of the attraction between the grounded target and the electrostatically charged fluid droplets. In other words, the droplets are uniformly drawn to the target by attraction due to the voltage difference between the droplets and the target.
An exemplary electrostatic dispensing nozzle assembly of the general type to which the present invention is directed is set forth in U.S. Pat. No. 5,209,410. Since, as will be apparent hereinafter, the nozzle assembly of the present invention can be supplied with fluid to be dispensed, mounted and supported in the same way taught in this patent, the teachings of this patent are hereby incorporated by reference.
Briefly, U.S. Pat. No. 5,209,410 describes an electrostatic nozzle assembly comprising a housing having a dispensing edge of a predetermined longitudinal length. The housing is made up of a base and a cap joined together to provide a substantially continuous slot adjacent the dispensing edge. Within the housing, and formed partially within the base and partially within the cap, there is a plurality of substantially hydraulically independent distribution chambers arranged serially along the longitudinal length of the housing. Each chamber is in fluid communication with the slot.
A conductive shim is located between the base and the cap and provides an electrical charge to the flowable material within the distribution chambers and the adjacent slot. This causes the flowable material to be electrostatically deposited from the nozzle assembly.
Each of the chambers of the nozzle is independently attached by a valved conduit to a source of the flowable material. Through use of the valves, the flowable material can be selectively supplied to individual distribution chambers. As a result, the dispensing process along the longitudinal length of the nozzle slot can be controlled, as desired. The nozzle can be adjusted for various widths of application without changing the structure of the nozzle. The distribution chambers of the nozzle assembly of this patent are each of a delta shape, which expands in width toward the dispensing edge.
The nozzle is provided with appropriate support means which also accommodates the valved conduits for the flowable material as well as electrical connection means by which the shim is connected to an appropriate voltage source.
The nozzle of U.S. Pat. No. 5,209,410 performs well, electrostatically dispensing the flowable material onto a target in a controllable and uniform manner. The nozzle is capable of serving both as a "shoot-down" nozzle (i.e. a nozzle aimed at a downward angle), and as a "shoot-up" nozzle (i.e. a nozzle directed at an upward angle).
Since the use of electrostatic nozzle assemblies has become so widespread, and since the applications in which they are used are so many and varied, a problem arises when a particular application requires the ability to spray fluids at a very low flow rate or at a high flow rate. Prior art electrostatic nozzle assemblies are characterized by liquid flow rate (or application rate) limitations, which cannot be exceeded.
As a result of this, the prior art has devised a number of nozzles, each designed for a particular flow rate range. Commonly assigned co-pending application Ser. No. 08/275,652, filed Jul. 15, 1994, U.S. Pat. No. 5,441,204, in the names of Daniel R. Tappel and Frederick R. Wichmann, and entitled ELECTROSTATIC FLUID DISTRIBUTION NOZZLE, teaches a nozzle assembly capable of reliable and uniform distribution across the width of the nozzle outlet opening of the fluid being dispensed, the nozzle operating at a relatively low flow rate. The teachings of this co-pending application are incorporated herein by reference.
Briefly, the co-pending application describes a nozzle comprising a nozzle body having a base and a cap. The nozzle has an inlet for receiving fluid at a relatively low pressure, and an elongated outlet opening enabling distribution of the fluid. A conductive shim assembly, located between the base and cap, provides an electrical charge to the fluid, enabling electrostatic distribution from the nozzle. The shim assembly is made up of two or more plates which, when appropriately stacked between the base and the cap define a distribution chamber comprising a plurality of distribution channels providing discrete pathways within the nozzle for distributing and directing the fluid from the inlet to predetermined dispensing points along the outlet opening. The pathways have substantially equal flow characteristics resulting in equal spraying characteristics along the length of the outlet opening. The distribution channels are part of the overall distribution chamber provided within the nozzle for receiving conductive fluid and directing it to the outlet opening in a uniform and consistent manner.
The present invention is based upon the discovery that a single electrostatic spray nozzle can be devised which will provide a full range of flow rate characteristics from very low flow rates down to about 0.04 cc/min/6 inches of nozzle with extremely even distribution to a high flow rate in excess of about 200 cc/min/6 inches of nozzle with extremely even distribution. The nozzle is provided with a plurality of shims, the selection of which will provide the flow characteristics desired. The nozzle is also of greatly simplified construction.